Such an arrangement is disclosed in U.S. Pat. No. 4,068,277.
In order to protect equipment connected to a telecommunication line against overvoltage, a primary protector and a secondary protector connected in parallel therewith are usually provided. The primary protector consists of, for example, a gas discharge tube or a carbon block. In the event of overvoltage such a protector reduces the line voltage to, for example, 180 V, after said voltage has first exceeded a threshold value of, for example, 400 V. However, actuation of this protector is subjected to a not insignificant turn-on time, in which the line voltage may still rise in certain circumstances to, for example, 1000 V. At the same time this protector does not offer a protection against voltages (for example 220 V) which are below the above-mentioned threshold value but which may still produce currents in the order of magnitude of 1 A. Currents of this magnitude may still cause damage. The secondary protector is intended to obviate the first-mentioned disadvantage of the primary protector. The conventional secondary protector comprises a rapidly switching voltage-limiting element such as, for example, a Zener diode or a voltage dependent resistor (VDR). This secondary protector is used to limit the line voltage during the switch-on period of the primary protector to, for example, 200 V. Thus, the protection against voltages induced by lightning is generally satisfactory. However, a short-circuit to mains, or a voltage induced from mains of the same order of magnitude as a result if unbalance in the mains supply, may cause destruction by overheating of the secondary protector of the fault persists for a period of, for example 0.1 seconds. This is of course unwanted. The problem in the existing situations is that the said power disturbances result in the destruction of the secondary protector. From the above-mentioned U.S. Pat. No. 4,068,277 it is known to avoid destruction by switching-off the line on which the disturbance occurs in the event of a power disturbance of, for example, 1 s or longer. The voltage-limiting circuit in said Patent Specification is a gas discharge tube. A heating coil which in the event of a power disturbance interrupts the contact between the said line and the gas discharge tube via a thermally operated switch is connected in series with said gas discharge tube.
A disadvantage of the said arrangements is that the heating coil and the voltage-limiting element may be provided in different locations and consequently may operate in different ambient temperature conditions, so that situations may occur in which overheating of the voltage-limiting element does not or not quickly enough result in switch-off of the line. A further disadvantage of the said arrangement is that a short circuit in or across that heating coil is possible; in that case overheating of the voltage-limiting element does not result in switch-off of the line.